REV. A

ADP3158/ADP3178

–8–

There are many useful references for quickly designing a power

inductor. Table II gives some examples.

Table II. Magnetics Design References

Magnetic Designer Software

Intusoft (http://www.intusoft.com)

Designing Magnetic Components for High-Frequency DC-DC

Converters

McLyman, Kg Magnetics

ISBN 1-883107-00-08

Selecting a Standard Inductor

The companies listed in Table III can provide design consul-

tation and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847) 639-6400

http://www.coilcraft.com

Coiltronics

(561) 752-5000

http://www.coiltronics.com

Sumida Electric Company

(408) 982-9660

http://www.sumida.com

The required equivalent series resistance (ESR) and capacitance

drive the selection of the type and quantity of the output capaci-

tors. The ESR must be small enough to contain the voltage

deviation caused by a maximum allowable CPU transient cur-

rent within the specified voltage limits, giving consideration also

to the output ripple and the regulation tolerance. The capaci-

tance must be large enough that the voltage across the capacitor,

which is the sum of the resistive and capacitive voltage deviations,

does not deviate beyond the initial resistive deviation while the

inductor current ramps up or down to the value corresponding

to the new load current. The maximum allowed ESR also repre-

The cumulative errors in the output voltage regulation cuts into

load regulation this relates directly to the ESR. When consider-

ing dc load regulation, this relates directly to the programmed

output resistance of the power converter.

Some error sources, such as initial voltage accuracy and ripple

voltage, can be directly deducted from the available regulation

window, while other error sources scale proportionally to the

amount of voltage positioning used, which, for an optimal design,

should utilize the maximum that the regulation window will allow.

The error determination is a closed-loop calculation, but it can

be closely approximated. To maintain a conservative design while

avoiding an impractical design, various error sources should

be considered and summed statistically.

The output ripple voltage can be factored into the calculation by

summing the output ripple current with the maximum output

current to determine an effective maximum dynamic current

change. The remaining errors are summed separately according

to the formula:

VV

V

k

I

II

k

k

kk

mV

WIN

VID

VID

O

OO

RCS

CSF

RT

EA

=×

×

+

+ 







++

















=

(

–

)

–

∆

∆

2

1

2

95

2

2

22

(5)

= 8% accounts for the IC current loop gain tolerance including

The remaining window is then divided by the maximum output

current plus the ripple to determine the maximum allowed ESR

and output resistance:

RR

V

II

mV

AA

m

E MAX

OUT MAX

WIN

OO

()

()

.

==

+

=

+

=

∆

Ω

95

15

3 8

5

(6)

The output filter capacitor bank must have an ESR of less

than 5 m

Ω. One can, for example, use five ZA series capacitors

from Rubycon which would give an ESR of 4.8 m

Ω. Without

ADOPT voltage positioning, the ESR would need to be less than

3 m

Ω, yielding a 50% increase to eight Rubycon output capacitors.

As long as the capacitance of the output capacitor is above a

critical value and the regulating loop is compensated with ADOPT,

the actual value has no influence on the peak-to-peak deviation

of the output voltage to a full step change in the load current.

The critical capacitance can be calculated as follows:

C

I

RV

L

A

m

HmF

OUT CRIT

O

EOUT

()

.

..

=

×

×

=

Ω×

×µ =

15

51 7

15

26

(7)

The critical capacitance for the five ZA series Rubycon capaci-

tors is 2.6 mF while the equivalent capacitance is 5 mF. The

capacitance is safely above the critical value.